I did have a read of those links (blush).
I think this is probably important to keep in mind:
"If the cable temperature can be kept relatively low (50–60°C for a 50% usage situation), the rate of migration will be slow and will not cause a problem over 50 years."
So Aaron is completely right that there's very little additional cost in sticking a straw down to insulate the wires so you may as well as intermittent shorts that develop would be maddening to debug (plus it makes your wires much easier to thread). However if your layout is used anywhere near close to 50% of the time at temperatures near 50-60deg and lasts for over 50 years then my hat goes off to you (although it's not entirely clear to me whether it's just surface area contact that counts and what effect temperature has on the migration rate).
Lets assume you've got a large layout and you're carrying 10A in a 12AWG/2mm main bus. From my back of the napkin (where I took a bunch of shortcuts but from memory it's not way off) that's going to to radiate less than 0.5C over ambient. And your main bus likely hangs below the foam anyway and obviously the current and heating of individual feeders should be much lower and carry current a lot less often (except possibly in a yard with a bunch of sound locos idling).
That's not to say that certain non-building code foam may be a little more reactive to PVCs (e.g. your fruit packing material white foam) but that's just speculation.
I'm now a little interested in whether the standard 2 and 3 cable jacket used in the US are migration resistant as Styrofoam has been used in wall insulation for well over 30 years. And it's common practice to embed power cables in it and I've never once heard of a short due to this. It wasn't really an issue for me when I was reading up on the code as we have to use rigid metal conduit in the Chicago area (although that's a fire code, rather than a building code reg, to address fires started by mice and rats chewing power cables). I also had to remove a switch from the layout recently that was installed around 7 years ago and looked very closely at the feeder droppers - I couldn't observe any deterioration in the jackets.
However, as Aaron said, the cost of protection is pretty low.